The present invention pertains to the field of austenitic alloys, i.e., alloys with a predominately face-centered cubic crystal structure. It is especially concerned with solid solution strengthened, non-magnetic, nickel-base, austenitic alloys having a relatively low thermal expansion coefficient and precipitation hardening, non-magnetic, nickel base, austenitic alloys also having a relatively low thermal expansion coefficient. It furthermore pertains to such alloys, with thermal expansions comparable to ferritic steels, and which have good resistance to stress corrosion cracking in caustic and chloride environments.
The thermal expansion coefficient (.alpha.) of materials in various engineering designs becomes especially important whenever a particular component involves more than one material and a fluctuation of temperature during on-and-off operations. Frequently, mismatch in the .alpha. of dissimilar metals develops thermal shock or thermal stress at the joint. A prolonged exposure of the component under sustained thermal stress and an aggressive environment may cause stress corrosion cracking (SCC). Often a geometric incompatibility develops in the close fitting of two dissimilar metals due to mismatch in .alpha.. This inhibits the use or affects and reliability of materials in many combinations that would otherwise be desirable.
The thermal expansion coefficient of the materials utilized in various component designs can become important in engineering structures such as steam turbines, gas turbines, boilers, nuclear reactors, fast breeder reactors, heat exchangers, pressure vessels, coal gasifiers, fluidized bed combustors, and vessels for production of synthetic fuels and petroleum products.
In steam turbines and heat exchangers, for example, many situations arise where an austenitic steel must be joined to or used in conjunction with, a ferritic steel. There are components in these systems typically being cycled from room temperature to an operating temperature in the range of 850.degree. to 1050.degree. F. where the typically large difference in the thermal expansion coefficients of the austenitic and ferritic materials gives rise to a variety of problems. For example, AISI 316 austenitic stainless steel pipe having an .alpha. (average thermal expansion coefficient over the temperature range of room temperature to 1000.degree. F.) equal to 10.2.times.10.sup.-6 in./in./.degree.F. is welded to ferritic P22 (ASTM A387-Grade 22) steel pipe (2.25 w% Cr, 1 w% Mo, 0.5 w% Mn, 0.4 w% Si, 0.15 w% C, balance Fe) having an .alpha. equal to 7.7.times.10.sup.-6. The transition joint weld is made using an austenitic stainless steel (16 w% Cr, 8% Ni, 2 w% Mo, balance Fe) weld metal having an .alpha. very similar to that of the 316 steel. In service, joints of this design are susceptible to the diffusion of carbon out of the P22 HAZ (heat affected zone) and into the 16 Cr--8 Ni--2 Mo stainless weld metal, where it precipitates. This loss of carbon from the P22 HAZ creates a plane of weakness which is susceptible to premature cracking under the stresses created in service due to the differential thermal expansion combined with pressure and bending stresses.
One attempt to resolve this problem has involved using an Incoloy 800 alloy (a trademark of the International Nickel Company for a nickel-chromium-iron base alloy) transition piece. This Incoloy 800 transition piece is welded at one of its ends to the 316 steel by a 16-8-2 weld, and welded at its other end using Inconel Filler Metal 82 (also a trademark of INCO and which meets the American Welding Society A5.14 Class ERNiCr-3 requirements) to the P22 ferritic steel. This joint design also produces carbon diffusion out of the P22 HAZ. Although the Incology 800 has an intermediate thermal expansion coefficient (.alpha..sub.(RT-1000.degree. F.) =9.3.times.10.sup.6 in./in./.degree.F.), reducing thermal expansion stresses somewhat at the austenitic/ferritic interface, a high level of stress is still present at this weakened, critical location.
Other specific examples in which the difference in thermal expansion is important include austenitic weld cladding on a ferritic base or bimetallic plate or pipe.
In addition to the above welding problems, the difference in thermal expansion between austenitic steels and ferritic steels causes difficulty in mechanical joints with the control of interference fits, clearances or stresses, where austenitic and ferritic members interact.
Furthermore, heavy section turbine components made from austenitic stainless steels, which have high thermal expansion coefficients and low thermal conductivity can be susceptible to thermal shock (or cracking) as a result of the cyclic operation of the turbine. Steam turbine parts in which thermal shock may be a consideration include valves, steam chests, nozzle blocks, and casings.
A specific example of the importance of matching thermal expansion coefficients is provided by the following example. The horizontal joints of high temperature steam turbine casings are held together by a large number of bolts and studs. Two types of bolting materials which have been used are a 12% Cr martensitic stainless steel (Type 422) whose thermal expansion coefficient (.alpha.) is less than the ferritic casing made of a 21/4% Cr-1% Mo ferritic steel casting and a nickel-base, precipitation hardened superalloy, whose thermal expansion coefficient is higher than the ferritic flange of the casting.
______________________________________ Thermal Expansion Coefficient, .sup.--.alpha., Room Temperature to 1000.degree. F. Material (538.degree. C.) .times. 10.sup.-6 in/in/.degree.F. ______________________________________ Type 422 6.5 21/4 Cr--1Mo 7.7 Ni--Base Superalloy 8.5 ______________________________________
Due to the thermal expansion and stress relaxation characteristics, the stresses in bolts made with these two materials respond differently during heating and in service. The initial hot stress of 12% Cr steel exceeds the cold set-up stress because of its low .alpha.; but in long time periods at temperatures above about 850.degree. to 900.degree. F. (455.degree. to 482.degree. C.), the stress gradually drops because of stress relaxation. In contrast, the initial hot stress of superalloy bolts is low because of the higher thermal expansion, but it remains constant with time because of its low relaxation. If the cold set-up stress of the superalloy is raised to increase the hot stress and improve the joint sealing efficiency, there is a possibility of plastic yielding of the bolt during thermal transients which occur during turbine starts. Since the steam first comes in direct contact with the casing, the flange heats up more rapidly than the bolt, thus temporarily increasing, rather than decreasing, the applied stress.
In the aforementioned high temperature applications discussed above it is therefore desirable to maintain .alpha. of the austenitic alloy as low as possible, and where an austenitic component must be joined to a ferritic component, .alpha. of the austenitic alloy should be as close as possible to .alpha. of the ferritic alloy. In addition to these considerations the alloy must also be resistant to stress corrosion cracking for usage in most elevated temperature turbine and heat exchanger applications.
Applicants have now developed a new class of austenitic, solid solution strengthened, non-magnetic, nickel-based alloys and a new class of austenitic precipitation hardening, non-magnetic, nickel-base alloys which can solve the specific problems and concerns discussed above. The range of the austenitic solid solution strengthened, nickel base alloys includes about: 12 to 21 w% Cr; an element for reducing the thermal expansion coefficient of the alloy, selected from the group consisting of Mo, W and their combinations and satisfying the following condition, 1.ltoreq.[wt% Mo+1/3(w% W)]&lt;7 w% and wherein the w% W does not exceed 12 w%, about 4 to 13 w% Fe; small but effective amounts of the desulfurizing agent, Mn; up to about 2.5 w% Si; up to 0.15 w% C; and up to 2 w% Co.
In addition the average thermal expansion coefficient, .alpha., of a solid solution strengthened alloy according to the present invention for the temperature range of room temperature to 1000.degree. F. is maintained below about 8.3.times.10.sup.-6 in./in./.degree.F. by balancing the composition of the alloys within the class to satisfy the following condition: ##EQU1##
Preferably the solid solution strengthened alloys according to the present invention are resistant to stress corrosion cracking in elevated temperature aqueous chloride ion and caustic solutions.
Preferably the solid solution strengthened alloys according to the present invention have an .alpha..sub.(RT-1000.degree. F.) less than about 8.1.times.10.sup.-6 in./in./.degree.F., and more preferably, less than 8.0.times.10.sup.-6 in./in./.degree.F.
It is also preferable that molybdenum and tungsten contents satisfy the following condition: 2.5 wt%&lt;[w% Mo+1/3(w% W)]&lt;7 w% but wherein the w% W does not exceed 12 w%.
The range of austenitic precipitation hardening, nickel base alloys according to the present invention includes: about 12 to 21 weight percent chromium; a concentration of molybdenum and/or tungsten, such that the sum of the weight percent molybdenum and one half the weight percent tungsten is between about 2 and 8 weight percent and wherein the weight percent tungsten is less than 12 weight percent; up to 13 weight percent iron; less than 0.15 weight percent carbon; less than 0.5 weight percent manganese; less than 1.0 weight percent silicon; less than 2.0 weight percent cobalt; 1.5 to 4.0 total weight percent of aluminum plus titanium and 0.7 to 4.5 total weight percent of columbium and/or tantalum.
In addition, the average thermal expansion coefficient, .alpha., of precipitation hardened alloys according to the present invention, for the temperature range of room temperature to 1000.degree. F., can be maintained below about 8.0.times.10.sup.-6 in/in/.degree.F. by balancing the composition of the alloys within this class to satisfy the following condition: ##EQU2##
Preferably the precipitation hardened alloys according to the present invention have an .alpha..sub.(RT-1000.degree. F.) of less than 7.9.times.10.sup.-6 in/in/.degree.F.
It is also preferable that iron content of the precipitation hardened alloys be between about 8 and 12 w% to optimize resistance to stress corrosion cracking.
Preferably, the precipitation hardening alloys according to the present invention are resistant to elevated temperature stress corrosion cracking in aqueous chloride and/or caustic contaminated environments.
These and other aspects of the present invention will become more clearly apparent upon review of the following detailed description of the present invention in conjunction with the figures which are briefly described below: